1. Field of the Invention
The invention relates to high temperature electrochemical cells, and to a method of forming a high temperature, dense, substantially gas-tight, electrically conductive interconnection layer on an electrode of an electrochemical cell. More particularly, the invention provides a method of closing pores in a porous plasma sprayed interconnection layer bonded to an electrode of a high temperature solid oxide electrolyte electrochemical cell, thereby forming a substantially gas-tight interconnection.
High temperature solid oxide electrolyte electrochemical generator devices are disclosed, for example, in Isenberg U.S. Pat. Nos. 4,395,468 and Isenberg U.S. Pat. No. 4,490,444. Such electrochemical generator devices comprise a plurality of elongated, typically annular, electrochemical cells or fuel cells which convert chemical energy into direct-current electrical energy. The fuel cells can be interconnected in series to provide a desired voltage and/or in parallel to provide a desired current capacity.
Each fuel cell typically includes an optional porous support tube of calcia stabilized zirconia. A porous annular air electrode or cathode generally surrounds the outer periphery of the support tube. The air electrode can be made with doped oxides of the perovskite family, such as, for example, lanthanum manganite (LaMnO.sub.3). A dense layer of gas-tight solid electrolyte, typically yttria stabilized zirconia (ZrO.sub.2), substantially surrounds the outer periphery of the air electrode. A porous fuel electrode or anode, typically of nickel-zirconia cermet or cobalt-zirconia cermet, substantially surrounds the outer periphery of the solid electrolyte. Both the solid electrolyte and the outer electrode, or, in this case the fuel electrode, are discontinuous to allow for inclusion of an electrically conductive interconnection material providing means to connect adjacent fuel cells. A selected radial segment of the air electrode, for example, is covered by the interconnection material. The interconnection material may comprise a doped lanthanum chromite (LaCrO.sub.3) film. The generally used dopant is Mg, although other dopants such as Ca and Sr have been suggested. The dopant serves to enhance the conductivity of the lanthanum chromite p-type conductor.
Various methods have been used to apply the interconnection material to the air electrode. Conventionally, both the electrolyte and the interconnection material are applied to the surface of different selected portions of the air electrode by a modified electrochemical vapor deposition process, at temperatures up to 1450.degree. C., with vaporized halides of zirconium and yttrium used for the electrolyte and vaporized halides of lanthanum, chromium, magnesium, calcium or strontium for the interconnection material, as taught in Isenberg U.S. Pat. Nos. 4,597,170 and Isenberg, et al. U.S. Pat. No. 4,609,562. The fuel electrode, typically nickel-zirconia cermet, is conventionally applied on top of the electrolyte, or grown on the electrolyte, by an electrochemical vapor deposition process, wherein nickel particles are anchored to the electrolyte surface by a vapor deposited skeleton of electrolyte material, as also taught in Isenberg, et al. U.S. Pat. Nos. 4,582,766 and Isenberg U.S. Pat. No. 4,597,170.
It has been found that there are certain thermodynamic and kinetic limitations in doping the interconnection material from a vapor phase by a vapor deposition process at temperatures between about 1300.degree. to 1450.degree. C. The vapor pressures of calcium chloride and strontium chloride, for example, are low at vapor deposition temperatures, and thus the transport of these dopants to the reaction zone at the surface of the air electrode is difficult. Therefore, magnesium has been used as the primary dopant for the interconnection material. However, magnesium doped lanthanum chromite has a substantial thermal expansion mismatch with the materials of the air electrode and electrolyte. For example, La.sub.0.99 Mg.sub.0.01 CrO.sub.3 has a thermal expansion mismatch of 12-14%. Additionally, halide vapors at 1300.degree. to 1400.degree. C. in a reducing atmosphere at partial pressures of O.sub.2 less that 1.times.10.sup.-4 atm can interact with the air electrode during the initial period of interconnection application. This may cause the air electrode constituents, such as manganese, to leach into the interconnection material and increase resistivity, causing problems in the electrochemical generator output. Also, the long term stability of interconnection layers made by electrochemical vapor deposition techniques remains questionable even though these techniques initially form substantially leak tight interconnection layers.
Adequately doping the interconnection material with Ca, Sr and other dopants identified in the Isenberg patents could reduce the thermal expansion mismatch problem with the air electrode and electrolyte material, but is constrained by kinetic and thermodynamic limitations of the electrochemical vapor deposition process.
Ruka U.S. Pat. No. 4,631,238 attempts to solve potential interconnection thermal expansion mismatch problems between the interconnection material, electrolyte, electrodes and support materials. Ruka teaches cobalt doped lanthanum chromite, preferably also doped with magnesium, for example, LaCr.sub.0.93 Mg.sub.0.03 Co.sub.0.04 O.sub.3, as a vapor deposited interconnection material using chloride vapors of lanthanum, chromium, magnesium and cobalt. Ruka similarly suffers from some kinetic and thermodynamic limitations.
Other methods of making doped LaCrO.sub.3 interconnection materials have been tried. Bowker, et al. U.S. Pat. No. 4,861,345 in a different approach, deposits particles of LaCrO.sub.3, doped with Mg, Sr, Ca, Ba or Co and coated with calcium oxide (CaO) or chromium oxide (Cr.sub.2 O.sub.3), on an air electrode. The doped LaCrO.sub.3 particles are solid-state sintered at high temperatures of about 1300.degree. C. to 1550.degree. C. Here, the coatings on the particles aid in sintering by providing a liquid phase which diffuses into the doped LaCrO.sub.3 structure. However, sintering the doped particles of interconnection material at high temperatures of about 1300.degree. C. to 1550.degree. C. to make a leak tight interconnection film, and then bonding it to the air electrode can result in several problems. These problems include Mn leaching from the air electrode into the interconnection material and diffusion of interconnection material dopant into the air electrode.
Pal, et al. U.S. Pat. No. 4,895,576, in another approach, teaches forming a layer of metal oxide particles, e.g., Ca, Sr, Co, Ba and Mg, on the interconnection portion of an air electrode, heating the air electrode with the deposited layer to about 1300.degree. C. to 1450.degree. C., and vapor depositing a skeletal structure of lanthanum chromite interconnection material around and between the metal oxide particles at about 1300.degree. C. to 1450.degree. C. in the presence of metal halide vapors comprising lanthanum, chromium, and magnesium chlorides. The metal oxide deposit is incorporated into the lanthanum chromium oxide structure as it grows thicker with time on top of the air electrode. Pal then anneals the unit at about 1100.degree. C. to 1400.degree. C., whereby metal ions of the metal oxide particles diffuse into the bulk of the lanthanum chromite interconnection material. This process requires a long annealing step in addition to deposition steps, to maximize conductivity by distributing the dopant across the lanthanum chromium oxide film. Some of the dopant can diffuse into the air electrode at such high temperatures.
None of these proposed solutions solves all the problems of thermal expansion mismatch, Mn leaching from the air electrode, dopant leaching from the interconnection material, and problems associated with doping Ca, Sr, Co, Ba and other materials by vapor deposition. Thus, none is optimally effective for providing a uniformly thick, durable, leak tight, well bonded, electrically conductive interconnection on a variety of substrates in a simple and economical fashion. Many of the problems, including the leak tightness of the interconnection material, are inherent in the process used to form the interconnection material. There is a need for a method of forming an interconnection layer on an electrode of an electrochemical cell, which provides a highly doped, substantially gas-tight, electrically conductive interconnection material without the associated problems.
Plasma arc spraying and flame spraying, i.e., thermal spraying or plasma spraying, are known film deposition techniques. Plasma spraying involves spraying a molten powdered metal or metal oxide onto the surface of a substrate using a thermal or plasma spray gun. Coker, et al. U.S. Pat. No. 4,049,841 generally teaches plasma and flame spraying techniques. Muehlberger U.S. Pat. Nos. 3,823,302 and 3,839,618 generally teach plasma spray guns.
Plasma spraying is a technique that has been used for fabrication of components, other than the interconnection material, in high temperature, solid oxide electrolyte electrochemical cells. Plasma spraying, however, has not been effectively used for the fabrication of the interconnection material. Attempts to plasma spray an interconnection material on an electrode structure of an electrochemical cell have been found to remain porous after cell processing, and, further, to lead to intermixing of the fuel and oxidant during cell operation. Gas can pass through the structure, whereas prior vapor deposition and sintering techniques provided a closed pore structure. Such intermixing of fuel and oxidant resulting from leakage through the interconnection microstructure is detrimental to electrical power generation, and also to the stability of the cell. Accordingly, the efficiency and the life expectancy of the electrochemical cell are substantially reduced. It is desirable for long term electrochemical fuel cell operation and component stability to provide a substantially leak tight interconnection.
None of these references teach or suggest effectively plasma spraying an interconnection material, and providing a plasma sprayed interconnection that is substantially leak tight and highly electrically conductive. There is a need to provide a dense, substantially gas-tight, highly doped, electrically conductive interconnection material in an electrochemical cell. There is a further need to provide a dense, gas-tight, interconnection material in an electrochemical cell that is substantially free of pores, at least at the surface using plasma spraying and densification techniques. The invention is directed to a solution to these problems by providing a method to densify or close the pores of a plasma sprayed interconnection layer, thereby forming a gaseous diffusion barrier and, accordingly, a substantially leak tight interconnection, while also providing a highly doped, electrically conductive interconnection.